Magnetic recording of both analog and digital information has been known and practiced in the art of audio reproduction and computer secondary storage. In magnetic recording, moving media such as a tape or disk platter may be coated with a ferro-magnetic oxide or metal coating. Information may be recorded with an electrically biased head which may be modulated with an information signal and which, in response, may generate a modulated flux field. Since a recording head may be in close proximity to a section of recording media, flux changes may cause ferro-magnetic particles in recording media to align accordingly. Conversely, information may be read from media by sensing alignment of flux fields representing recorded information.
For information comprising digital data, recording heads may write and read flux reversals. Flux reversals, in conjunction with timing and coding information, may indicate presence or absence of a data bit. Methods for decoding flux reversals corresponding to recorded information and synchronizing bit recovery timing are well known in the art. In conventional head based recording, data density is based on parameters such as read head size and sensitivity. Also, retentivity--tendency of magnetic materials to retain residual magnetic flux and resist changes in flux alignment--of head materials may limit the rate at which a conventional head could sense a flux reversal and thereby limit maximum areal density readable by a head. Areal density refers to number of bits of information capable of being recorded within a linear measure of a particular magnetic recording medium.
Conventional methods to increase areal density have included using smaller, thin film inductive read heads. Inductive heads generate an electrical signal when a magnetic field moves in relation to it. A resulting induced electrical signal may be used directly to recover information from media. Inductive heads like virtually all inductive devices have the disadvantage of overshoot related to inductive discharge when faced with changing signal polarity. Additional circuitry may be necessary to remove undesirable artifacts associated with overshoot from the information signal. Other mechanical and electrical disadvantages may be associated with inductive heads. For example, relative manufacturing complexity of inductive heads may lead to electrical inconsistency giving rise to problems with electrical calibration of heads and/or read circuits.
In contrast, thin film Magneto Resistive (MR) heads or transducers may be less likely to exhibit significant differences from one head to another because their properties derive from material properties rather than head construction. MR heads may comprise a layer of permalloy material continuously biased with a sense current. When a magnetic field moves in proximity to an MR head, a change in resistance and corresponding change in sense voltage occurs based on a constant sense current. Data may be read from magnetic media by decoding a sense current signal modulated by magnetic flux transitions representing information recorded on a section of media. For greater sensitivity, so-called shielded MR heads may have a layer of magnetic material between the head element and the media.
Other limitations affecting recording density of information is the Nyquist limitations of the channels used to read and write data. The present invention deals primarily with problems associated with read channels, however Nyquist limitations are known in the art to affect maximum rates associated with any information channel. On a sampled channel, the Nyquist frequency is the minimum sampling frequency required to reproduce the original information from the sampled information. Nyquist criteria and problems associated with Nyquist rates are well known and appreciated in the art. Partial Response signalling or correlative coding may be used to bandlimit transmission over a channel.
In addition to bandlimiting, correlative coding avoids Nyquist limitations by introducing cross correlation and a limited amount of intersymbol interference. By removing symbol correlative independence, higher rates may be achieved than possible with Nyquist limitations associated with more highly correlated symbols. Tradeoffs may involve an additional decoding step as a Viterbi decoding step or other such decoding step when information on a Partial Response channel is received. Processing tradeoffs may be necessary since removing correlative independence by preceding and correlative filtering may have the effect of introducing Gaussian products to coded information which must eventually be removed. Correlative coding is described in more detail in a text entitled "Communication Systems", A. Bruce Carlson, .COPYRGT.1986, McGraw-Hill and is incorporated herein by reference.
Another significant disadvantage of conventional thin film inductive heads may be an inability of such inductive heads to read data when media linear velocity slows below a threshold value. Reduced linear velocity may be most acute near the disk spindle or center. Even though spindle rotational velocity remains constant, linear velocity of a given point on a spinning disk platter decreases closer to the spindle. In an inductive head, generation of a read signal relies on relative motion between an inductive read head and flux fields recorded on a section of media. As linear velocity decreases, read signal characteristics change because of changes in relative motion and thus induction between media and head. In an extreme case, with no relative motion, no read signal would be generated from an inductive read head. Since magneto-resistive properties are independent of relative motion, MR heads may not exhibit such problems reading data from disk sections near the spindle with lower linear velocities.
MR heads have significantly increased areal density capability of disk drives using magnetic media. Hard disk drive capacity has been increasing proportional to increases in areal density achievable by manufacturers, while price per Mbyte has decreased in like proportion. By achieving higher areal density, more data may be stored on the same platter. With the exception of MR head circuitry, virtually all other disk drive components may remain the same. For commercial viability however, gains in areal density due to MR head technology must be accomplished with no significant increase in bit error probability.
Along with advantages of MR head use related to increased areal density and compact packaging come problems associated with increased susceptibility to errors caused by thermal asperities (TAs). TAs refer to recording signal anomalies caused by contact between an MR head and rough spots or asperities on media surfaces. Asperities may be caused by manufacturing defects or flecks of metal oxide on recording media surfaces. TA related errors may be caused by rapidly rising MR head temperature due to momentary contact with an asperity. A rise in temperature may change head resistance causing a corresponding transient in output signal voltage. Since information read from magnetic media may correlate to MR head output voltage, transients in MR head voltage may correlate directly to errors. When severe, TA induced burst errors may be unrecoverable since number of resulting errors exceeds the syndrome or capability of error correcting codes (ECC) used in read processing circuits. TAs are described in more detail in a publication entitled "Magneto-Resistive Head Thermal Asperity Digital Compensation", R. L. Galbraith, et al, IBM Storage Systems Product Division, Feb. 17, 1992 incorporated herein by reference.
One particularly troublesome problem with TAs making them difficult to detect and correct may be their timing. TA related transients in MR head output may occur with risetimes measured in nanoseconds. Fast risetimes may make TAs difficult to detect in a sufficient amount of time to invoke corrective measures. Moreover, with durations of several microseconds, TAs may cause continuous error bursts before dissipating. The amplitude of transients caused by TAs may be greater than twice peak amplitude of the MR head read signal.
Galbraith, et al describes an Analog to Digital Converter (ADC) expanded headroom technique and a timing and gain correction hold technique. Galbraith's method may have the disadvantage of reduced signal-to-noise ratio when operated in an expanded headroom mode. Moreover, lack of control of TAs during hold periods when timing and control loop operation is suspended may cause a complete loss of read data synchronization and gain synchronization.
Ottensen, et al., U.S. Pat. No. 5,367,409, discloses even harmonic distortion compensation for digital data detection. Ottensen's method may have disadvantages associated with an expensive oversampling ADC and adder. Hardware costs associated with an oversampling ADC and adder coupled with an extra addition step may comprise disadvantages which distinguish the present invention from Ottensen.